Various systems and methods exist for determining the location of a device using wireless signals. One method for determining the location of a device is multilateration. Multilateration is used in civil and military applications, for example, to locate vehicles, aircraft, and mobile or stationary receivers or emitters of radio frequency (RF) signals.
In one form of multilateration, a transmitter emits an RF signal. The position of the transmitter is determined by receiving the transmitted signal at three or more synchronized receivers located at known locations. The signal includes a time code indicating a time at which the signal was emitted. The receivers include hardware and software for computing the time difference of arrival (TDOA) of the transmitted signal at each of the receivers. The TDOA is used to determine the distance between each receiver and the transmitter. An intersection of the three distances is determined. The intersection of the three distances indicates the position of the mobile communications device at the time the received signal was transmitted, though some error typically exists from signal noise, signal interference, clock error, signal echo, and signal reception errors.
In a second form of multilateration, a receiver receives RF signals emitted by three or more transmitters. Each of the signals includes a time code indicating a time at which the signal was emitted, and travels at a known speed. In some embodiments, each signal transmitter is at a known location. Alternatively, the location of the transmitter can be encoded in the emitted signal. At any rate, the receiver includes hardware and software for computing the time difference of arrival (TDOA) of each of the received signals. The TDOA is used to determine the distance between the receiver and each of the transmitters. An intersection of the three distances is determined to indicate the location of the receiver, though subject to errors similar to those mentioned above.
This second form of multilateration, i.e., determining the TDOA of three or more signals received at a receiver, is similar to the method used by Global Positioning System (GPS) receivers. GPS receivers rely upon RF signals transmitted by a constellation of satellites with known locations. A TDOA for each received GPS signal is determined, based upon an embedded time code, a known speed of the GPS signal, and a known location of the emitting satellite. The intersection of the three or more signals is determined. This intersect indicates the location of the GPS receiver. Many GPS receivers use a signal from a fourth or additional satellite, when available, to correct error in the determined location, for example, by correcting clock error, interference, and/or for purposes of determining the altitude of the receiver.
Over the past several years, the use of mobile communications devices and networks has increased. Some modern wireless communications networks now include devices and software for providing users with location based services (LBS) such as navigation. The location of mobile communications devices may be used to provide users with services and/or features tailored to a particular location, for example, navigation, emergency services and response, location based services, and other purposes. Network operators may use location information to analyze network infrastructure, usage patterns, and the like, and/or to monitor maintenance, planning, and operational issues, if desired.
In order to provide location based services, some mobile communications devices now include GPS capabilities, and/or rely upon other systems and methods, for example, triangulation, multilateration, and/or other TDOA techniques, Assisted GPS (A-GPS), satellite links, and interfacing with short- and long-range location beacons.
With respect to TDOA techniques, mobile communications devices can receive RF signals emitted by synchronized transmitters. In general, each transmitter emits reference signals used by cellular handsets to determine position in a manner substantially similar to the methods described above. The reference signals are often emitted on a determined schedule, for example, every 10 milliseconds (ms).
Although weather conditions can have measurable effects on the reference signal speed, reference signals are usually received from relatively short range at the frequencies used for cellular communication, compared to the distances traveled by GPS signals or other long-range communications system signals. In good weather conditions and flat terrain, the maximum range of cellular signals is about 45 miles. Outside of dense urban environments, however, cellular towers are often located within 1-2 miles of each other. Even under greatly varied conditions, the short travel range of the reference signals means that the condition-based speed variations are generally negligible.
Furthermore, given the speed and short range of the cellular signals, the TDOA of the cellular signals are typically expressed as extremely small fractions of a second. As such, the 10 millisecond pause between signal transmissions is generally adequate to prevent the handset from mistakenly relying upon more than one synchronized reference signal emission for any particular location determination operation.